1. Field of the Invention
The present invention relates to a communication system that comprises a plurality of nodes connected in common to two-wire transmission lines and more particularly to a fault-detecting device for detecting a fault such as a break, a short circuit, and the like of transmission lines.
2. Detailed Description of the Related Art
As shown in FIG. 1, in a prior art communication system, two-wire transmission lines 1, 2 are connected with transmission/reception circuits 31-3n at a plurality of nodes. All the transmission/reception circuits 31-3n comprise the same components. Positive potential Vcc (for example, 5 V) is supplied to one end of the transmission line 1 via a terminal resistor 4 and positive potential Vcc is supplied to the other end via a terminal resistor 5 in the same way. Ground potential Vg (for example, 0V) is supplied to one end of the transmission line 2 via a terminal resistor 6 and ground potential Vg is supplied to the other end via a terminal resistor 7 in the same way.
In the transmission/reception circuit 31, a two-way I/O filter 11 is connected to the transmission lines 1, 2 via a connector 12. Connecting terminals A1, A2 are provided for connecting the I/O filter 11 to the transmission lines 1, 2 and connecting terminals B1, B2 arranged as opposed to the connecting terminals A1, A2. A transmission signal is individually supplied to the connecting terminals B1, B2 via a non-inverting amplifier circuit 13 and an inverting amplifier circuit 14. In addition, bias circuits 17, 18 are connected to the connecting terminals B1, B2 of the filter 11 via AC coupling circuits 15, 16 which comprise a resistor and capacitor, respectively. Each of the signals provided by the bias circuits 17, 18 serves as a reception signal via a comparator 19.
Upon outputting the transmission signal, the signal is amplified by the non-inverting amplifier circuit 13 and amplified in an inverting manner by the inverting amplifier circuit 14 as well. Transmission signals opposite in phase to each other are supplied to the filter 11 from the non-inverting amplifier circuit 13 and the inverting amplifier circuit 14. The filter 11 serves as a low-pass filter to allow the transmission signals to pass individually therethrough. An output transmission signal from the non-inverting amplifier circuit 13 passes through the filter 11 and is thereafter supplied to the transmission line 2 as an information signal. An output transmission signal from the inverting amplifier circuit 14 passes through the filter 11 and is thereafter supplied to the transmission line 1 as an information signal.
On the other hand, the information signals, opposite in phase to each other and transmitted through each of the transmission lines 1, 2 are supplied to the filter 11. The filter 11 acts as a low-pass filter on each of. these information signals to output the signals to the AC coupling circuits 15, 16. Each of the AC coupling circuits 15, 16 extracts AC components of the information signals and supplies the components to the bias circuits 17, 18, respectively.
For example, as shown in FIG. 2A, consider the case where a signal A transmitted through the transmission line 1 and a signal B transmitted through the transmission line 2 vary as opposed in phase to each other. As shown in FIG. 2B, the bias circuit 17 applies a bias voltage to the information signal A to obtain a biased signal BIASA, while the bias circuit 18 applies a bias voltage to the information signal B to obtain a biased signal BIASB. As shown in FIG. 2C, the comparator 19 detects each of the output signals BIASA, BIASB from the bias circuits 17, 18 as a reception signal RX0.
When a break has occurred in the transmission line 1, only signal B is transmitted in the transmission line 2. Accordingly, as shown in FIG. 2D, the biased signal BIASA remains constant, whereas the biased signal BIASB, transmitted through the transmission line 2, to which a bias voltage has been applied changes like the signal B. The comparator 19 compares the constant biased signal BIASA and the biased signal BIASB to obtain a reception signal as shown in FIG. 2E. This holds true even when the transmission line 1 is grounded or when the transmission line 2 is broken or grounded.
Incidentally, no reception signals could be detected without the bias circuits 17, 18 when a break occurs in the transmission line 1 since the signals A, B to be inputted into the comparator 19 would have the waveforms shown in FIG. 2F.
A fault detecting device for detecting a fault such as a break or a short circuit or the like on the transmission lines 1, 2 comprises comparators 20, 21 and mismatch detecting circuits 22, 23. The comparator 20 compares the biased signal BIASA with a threshold value Vth. A high level output is obtained when the biased signal BIASA is equal to or less than the threshold value Vth, whereas a low level output is obtained when the biased signal BIASA is greater than the threshold value Vth. The output is supplied to the mismatch detecting circuit 22 as an individual reception signal RX1. The mismatch detecting circuit 22 reads, in phase with a sampling clock, each of the reception signals RX0, RX1 of the comparators 19, 20. The mismatch detecting circuit 22 provides a low level output when the levels of the read reception signals RX0, RX1 coincide with each other. On the other hand, when the levels of the reception signals RX0, RX1 do not coincide with each other, the mismatch detecting circuit 22 provides a high level output that shows that a fault has occurred on the transmission line 1.
Likewise, the comparator 21 compares the biased signal BIASB with the threshold value Vth. A low level output is obtained when the biased signal BIASB is equal to or less than the threshold value Vth, whereas a high level output is obtained when the biased signal BIASB is greater than the threshold value Vth. The output is supplied to the mismatch detecting circuit 23 as an individual reception signal RX2. The mismatch detecting circuit 23 reads, in phase with the sampling clock, each of the reception signals RX0, RX2 of the comparators 19, 21. The mismatch detecting circuit 23 provides a low level output when the levels of the read reception signals RX0, RX2 coincide with each other. On the other hand, when the levels of the reception signals RX0, RX2 do not coincide with each other, the mismatch detecting circuit 23 provides a high level output that shows that a fault has occurred on the transmission line 2.
In response to the high-level output showing a fault, for example, the transmission/reception circuit 31 activates fault corrective functions such as generating an alarm or stopping transmission and/or reception operation.
Other transmission/reception circuits 32-3n also have the same configuration and functions as those of the transmission/reception circuit 31. Incidentally, a device that detects a fault on a transmission line based on signal levels are disclosed in Japanese Patent Laid-Open Publications No. Hei 5-147479 and No.Hei 5-75629.
However, once it is detected that the level of the signal transmitted through the transmission line 1 or 2 is abnormal in such prior art fault-detecting device of a communication system, the device judges immediately that a fault has occurred in the transmission line 1 or 2. Accordingly, even a disturbance such as a noise that would never exert an adverse effect on the transmission/reception operation of the system would cause the device to judge that a fault occurred in the transmission line 1 or 2. This would cause the fault corrective functions to work unnecessarily.
In view of the aforementioned circumstances, an object of the present invention is to provide a fault-detecting device for a communication system that can judge correctly that a fault has occurred in two-wire transmission lines, which affects adversely with certainty the transmission/reception operation thereof.
A fault-detecting device of the present invention for a communication system using two-wire transmission lines for transmitting information signals opposite in phase to each other is characterized by comprising first comparator means for comparing magnitudes between levels of information signals inputted through each of the two-wire transmission lines to obtain a resulting value as a main reception signal; second comparator means for comparing magnitudes between a level of an information signal inputted through one of the two-wire transmission lines and a first threshold value to obtain a resulting value as a first individual reception signal; third comparator means for comparing magnitudes between a level of an information signal inputted through the other one of the two-wire transmission lines and a second threshold value to obtain a resulting value as a second individual reception signal; first mismatch detecting means for determining a mismatch between the main reception signal and the first individual reception signal at a predetermined timing and generating a first mismatch detection signal when the mismatch has occurred; first frequency determining means for generating a first fault detection signal indicating a fault in the one of the two-wire transmission lines in accordance with a frequency of occurrence of the first mismatch detection signal; second mismatch detecting means for determining a mismatch between the main reception signal and the second individual reception signal at the predetermined timing and generating a second mismatch detection signal when the mismatch has occurred; and second frequency determining means for generating a second fault detection signal indicating a fault in the other one of the two-wire transmission lines in accordance with a frequency of occurrence of the second mismatch detection signal.
According to such fault-detecting device of the present invention, since a main reception signal and a first individual reception signal have generally the same waveform when there is no fault on one of the transmission lines, a first mismatch detection signal is generated when a mismatch between the main reception signal and the first individual reception signal has occurred and determined at a predetermined timing. Then, a first fault detection signal that indicates the occurrence of a fault in the one transmission line is generated in accordance with the frequency of occurrence of the first mismatch detection signal. On the other hand, a second mismatch detection signal is generated when a mismatch between the main reception signal and a second individual reception signal has occurred and determined at a predetermined timing since the main reception signal and the second individual reception signal have generally the same waveform when there is no fault on the other one of the two-wire transmission lines. Then, a second fault detection signal that indicates the occurrence of a fault in the other transmission line is generated in accordance with the frequency of occurrence of the second mismatch detection signal. Accordingly, even when a mismatch between the main reception signal and the first or second individual reception signal is once detected, a fault detection signal is not immediately generated. Therefore, this makes it possible to judge correctly that a fault that exerts an adverse effect with certainty on the transmission/reception operation has occurred in two-wire transmission lines.
The first frequency determining means comprise a first counter for counting the number of times of occurrence of the first mismatch detection signal and generating the first fault detection signal when the counted number has exceeded a predetermined count value. On the other hand, the second frequency determining means comprise a second counter for counting the number of times of occurrence of the second mismatch detection signal and generating the second fault detection signal when the counted number has exceeded the predetermined count value.
Furthermore, the first frequency determining means comprise a first frequency-voltage converter for converting a frequency of occurrence of the first mismatch detection signal and fourth comparator means for generating the first fault detection signal when an output voltage of the first frequency-voltage converter has exceeded a predetermined voltage. On the other hand, the second frequency determining means comprise a second frequency-voltage converter for converting a frequency of occurrence of the second mismatch detection signal and fifth comparator means for generating the second fault detection signal when an output voltage of the second frequency-voltage converter has exceeded the predetermined voltage.